Potential mechanisms underlying the modest anti-tumour efficacy include the presence of multiple co-inhibitory ligands or receptors on cancer cells or T cells infiltrating tumours, compensatory upregulation of multiple immune checkpoints and other immunosuppressive mechanisms (e.g., Indoleamine 2,3-dioxygenase (IDO) and TGF-β) [5]. Here, IDO1 is linked to neoplasm.